1. Field of the Invention
This invention relates to a process for the preparation of a compound of the general formula: ##STR1## wherein R.sup.1 represents an optionally substituted hydrocarbyl group, R.sup.2 a methyl group or a hydrogen atom or R.sup.1 and R.sup.2, together with the carbon atom to which they are attached jointly form a cycloalkylidene group, each Hal a chlorine or bromine atom and R.sup.3 (a) a group --C(O)OM, in which M represents an alkali metal atom, (b) a group --C(O)OR.sup.4, in which R.sup.4 represents an optionally substituted alkyl or an optionally substituted cycloalkyl group, (c) a hydrogen atom or (d) a group R.sup.5 present in an acid of the general formula R.sup.5 OH.
2. Description of the Prior Art
Compounds of the general formula I in which R.sup.1 represents a substituted cyclopropyl group, such as 2-(2,2-dimethoxyethyl)-3,3-dimethylcyclopropyl group, are intermediates in the preparation of 3-aryloxybenzyl esters of substituted 2,2-dihalovinylcyclopropanecarboxylic acids. These esters--which are also called "synthetic pyrethroids"--have exceptionally good insecticidal properties while possessing a very low mammalian toxicity (see U.S. Pat. No. 4,024,163. This combination of properties makes them of considerable interest to the agrochemical industry, and much effort has been expended in finding economical routes for their preparation.
A formyl group in an organic compound, for example, in intermediates to synthetic pyrethroids, can be converted into a ##STR2## by reaction with sodium trichloroacetate in the presence of 1,2-dimethoxyethane, followed by dilution of the reaction mixture with water, as described in J. Org. Chem., 32, 2166-2171 (1967). This conversion is effected by stirring the reaction mixture at room temperature for 80 hours. However, even after such a long reaction time the conversion of the starting aldehyde is far from complete. Furthermore, the compound having the group ##STR3## should be isolated from the mixture obtained after dilution with water if this compound is to be reacted with water-sensitive compounds for further conversion.
The selectivity to a certain compound, expressed in a percentage, is defined as ##EQU1## wherein "a" is the amount of the starting compound (for example, aldehyde) converted into that certain compound and "b" is the amount of converted starting compound.
The applicants have repeated this known process at a temperature of 80.degree. C. and observed a high conversion of the starting aldehyde in less than one hour, but a low selectivity to the compounds having the group ##STR4## the selectivity to compounds formed as a result of aldol condensation being fairly high.
It has now been found that a rapid and high conversion of the starting aldehyde, the reaction time being usually between 1 and 30 minutes at ambient temperature, with a high selectivity to the compounds of formula I in which R.sup.3 represents the group --C(O)OM, can be obtained by carrying out the reaction in a highly polar, aprotic, inert solvent. Moreover, the compounds of formula I in which R.sup.3 represents the group --C(O)OM, being organic alkali metal carbonates, can be converted in usually high yield into, for example, compounds of formula I in which R.sup.3 represents a hydrogen atom, the group --C(O)OR.sup.4 mentioned sub (b) or the group R.sup.5 mentioned sub (d) hereinbefore.